1. Field of the Invention
The present invention relates to a dynamic pressure bearing device including a fixed portion and a rotating portion supported rotatably by this fixed portion via a dynamic pressure bearing portion, and to a manufacturing method and an assembly jig thereof, and is applicable, for example, not only to a disk-rotating-type memory such as a drive unit for a hard disk drive (HDD) and a digital versatile disk (DVD) but also to a polygon mirror and so on.
2. Description of Related Art
A dynamic pressure bearing device is generally so structured that a dynamic pressure is given to an operating oil which is filled in a gap between a rotating portion including a rotating shaft and a fixed portion by utilizing a rotational force of the rotating shaft, and this dynamic pressure brings the rotating portion into a levitated state relative to the fixed portion, thereby supporting the rotating shaft rotatably.
In this dynamic pressure bearing device, a dynamic pressure bearing portion which generates the dynamic pressure to support the rotating shaft is generally includes a journal bearing portion receiving a force acting in a direction perpendicular to the rotating shaft (a diameter direction) and a thrust bearing portion receiving a force acting in a direction along the rotating shaft (for example, refer to Japanese Patent Laid-open No. Hei 10-96421 and so on).
In a dynamic pressure bearing device in a miniature motor such as a spindle motor for an HDD, a width dimension of the gap which is formed between the rotating portion and the fixed portion when the rotating portion is supported in the levitated state relative to the fixed portion by the dynamic pressure generated in accordance with the rotation of the rotating shaft is generally set to about 2 μm to about 3 μm in both of the journal bearing portion and the thrust bearing portion, in consideration of the areas of facing surfaces thereof, load capacitance, viscosity of a fluid, and so on.
The width dimension of this gap is an important factor having a decisive influence on a levitating force, a loss torque, or bearing stiffness and variation in the width dimension of this gap causes a large variation in performance of the dynamic pressure bearing. Therefore, the deviation from a set value of the width dimension of the gap which is formed when the rotating portion is actually supported in the levitated state needs to be controlled within ±several tenths μm, and the deviation of this dimension is conventionally controlled by making constituent parts of the device highly precise and by screening and combining parts.
However, since an adhesive is used for fixing the constituent parts of the device due to lack of space to be used for fastening tools such as screws and it is difficult to control variation in thickness of the solidified adhesive to be small, it is difficult to control the variation in the width dimension of the gap in the dynamic pressure bearing portion, especially the gap in the thrust bearing portion, within a desired range.
Further, in this dynamic pressure bearing device, a fluid is filled as the operating oil constituting the dynamic pressure bearing portion, and a lubricating oil or a magnetic oil is used as this fluid. Here, a low-viscosity liquid which undergoes a small secular change is mainly used as the fluid used as the operating oil from the viewpoint of power consumption reduction, life elongation of the dynamic pressure bearing, and so on.
One of the objects of using the magnetic fluid is that outward dispersion of the operating oil is prevented by forming an appropriate magnetic circuit inside the bearing. Another object is that, when an ordinary lubricating oil is used as the operating oil in the HDD and so on, static electricity on a disk which is generated during the rotation of the disk cannot be transmitted from the rotating portion to the fixed portion due to no conductivity of the lubricating oil, but when, on the other hand, the magnetic fluid is used as the operating oil, the static electricity can be transmitted from the rotating portion to the fixed portion via the conductive magnetic fluid to be grounded so that the destruction of a read/write head due to electrostatic discharge is prevented.
When the magnetic fluid is used as the operating oil, a ferromagnetic field is formed in the vicinity of the boundary surface between the magnetic fluid and an external space and the magnetic fluid is held by the ferromagnetic field to prevent an outward flow of the magnetic fluid as the operating oil. At this time, as a magnet for forming the ferromagnetic field, a nylon-based neodymium magnet, a samarium-cobalt magnet, a ferrite magnet, or the like is generally used.
In both cases of using the lubricating oil and of using the magnetic fluid as the operating oil, a lipophobic agent is applied on the surfaces of members which are disposed in the vicinity of the boundary surface between the fluid and the external space to prevent the exudation thereof, thereby preventing the outward flow of the fluid. This is because a liquid on a metal surface with no lipophobic agent applied thereon causes an exudation phenomenon, and a part where the exudation phenomenon occurs serves as a passage through which the fluid leaks out, and a centrifugal force or the like further promotes the outward dispersion of the operating oil through the passage. Therefore, the prevention of the exudation of the operating oil is an important technique for the dynamic pressure bearing device which uses a liquid as the operating oil, and the lipophobic agent to be applied needs to adhere well to the metal surface, undergo small secular changes, and stably and continuously exert the function thereof.
Incidentally, the conventional dynamic pressure bearing device has the following disadvantages. Namely, as described above, the gap between the rotating portion and the fixed portion at the time when the rotating shaft is supported in the levitated state by the dynamic pressure bearing portion is generally set to about 2 μm to about 3 μm in the conventional dynamic pressure bearing device applied to the miniature motor such as the spindle motor for the HDD, and in this case, the variation in the gap has to be controlled to be ±several tenths μm or less in order to reduce variation in precision, a levitating force, power consumption, and so on of the rotating shaft. Therefore, it is necessary to demand a high precision for the dimension of each constituent member of the dynamic pressure bearing device.
Constituent members of the journal bearing portion are relatively easily manufactured since only the precision enhancement of the inner and outer diameters is what is required for these members, but since constituent members of especially the thrust bearing portion require variation reduction in height, it is very difficult to secure a required dimension precision for these constituent members in view of the precision of parts which is obtained by ordinary metal machining. Therefore, there exists a problem that this will be a cause of increasing the price of the dynamic pressure bearing device and a cause of preventing the dynamic pressure bearing device from being in wide use.
Moreover, the adhesive is used to fix the constituent parts due to the lack of space in the conventional dynamic pressure bearing device which is applied to the miniature motor such as the spindle motor for the HDD, as described above. However, since it is difficult to control the variation in thickness of the solidified adhesive to be small, there exists a disadvantage that it is difficult to control the variation in the gap in the dynamic pressure bearing portion, especially in the thrust bearing portion to be small.
Further, the magnetic fluid is used as the operating oil in order to prevent the fluid which is filled in the gap in the dynamic pressure bearing portion from flowing and dispersing to the external space in the conventional dynamic pressure bearing device as described above. However, the viscosity of the magnetic fluid which is made by dispersing metal particulates in a colloidal state into a lubricating oil using a surface active agent is generally high compared with the lubricating oil. This will lead to the increase in power consumption for rotating the rotating shaft, and since self-heating occurs in the magnetic fluid due to a shear received by the magnetic fluid in accordance with the rotation of the rotating shaft in the dynamic pressure bearing portion to promote oxidation or evaporation of the fluid, there exists a disadvantage that the life of the dynamic pressure bearing device using the magnetic fluid is short compared with that using the lubricating oil.
Moreover, when the magnetic fluid is used as the operating oil of the dynamic pressure bearing device, the nylon-based neodymium magnet, the samarium-cobalt magnet, the ferrite magnet, or the like is conventionally used as the magnet for forming the ferromagnetic field in the vicinity of the boundary surface between the magnetic fluid and the external space, as described above. However, the particle diameter of any of these magnets is 0.8 mm or more so that it is impossible to set the thickness or the difference between the inner and outer radius thereof to 0.5 mm or less. Therefore, since space for arranging these magnets cannot be secured in the dynamic pressure bearing device of the spindle motor for the disk-rotating-type memory which uses a disk with an outer diameter of, for example, 1.5 inches or less, it is difficult to manufacture a dynamic pressure bearing device which can constitute a microminiature spindle motor.
As described above, in the conventional dynamic pressure bearing device using the lubricating oil or the magnetic fluid as the operating oil, the lipophobic agent is applied on the constituent members of the device arranged in the vicinity of the boundary surface between the operating oil which is filled in the gap in the dynamic pressure bearing portion and the external space to prevent the outward exudation of the operating oil. However, the members on which this lipophobic agent is applied are made of metal to necessitate the application of the lipophobic agent on machined surfaces thereof. Incidentally, in metal machining, a cutting oil is generally used, and at this time, since a metal surface newly appearing as a result of working with a cutting tool is active, this metal surface is immediately bound with the cutting oil. Then, the cutting oil thus bound with the metal surface remains on the metal surface in a molecular level thickness and is not easily removed or exfoliated by chemical washing or the like. Accordingly, when the lipophobic agent is applied on the metal surface with which the cutting oil is bound, such a problem is caused that the lipophobic property is greatly lowered since adhesion between the lipophobic agent and the metal surface is low to easily cause the exfoliation of the lipophobic agent.